P Intrinsic N (PIN) Diodes are used commonly in an assortment of commercial and military applications as switches, phase shifters, and attenuators. Recently, a variety of multi-port PIN designs capable of operating at mmW frequencies have become available by integrating PIN diodes with passive elements on a common compound semiconductor substrate.
These new designs are representative of a type of device referred to herein as a “compound semiconductor device.” As used herein, this term refers to a device which comprises a compound semiconductor substrate upon which are disposed active and passive elements where are electrically coupled together, either directly or indirectly. As used herein, the term “compound semiconductor” refers to any non-silicon-containing compound of two or more elements having an electrical conductivity between that of a conductor and a dielectric. Common compound semiconductors include III-V and II-VI group compounds such as gallium arsenide (GaAs) and zinc sulfur (ZnS), respectively. Typically, a compound semiconductor does not comprise a group IV element. The term “active element,” as used herein, refers to an electrical component having at least two states between which there is no linear correlation, that is, the component switches between the states according to a step function. Examples of active elements include PIN diodes and transistors. Conversely, the term “passive element,” as used herein, refers to an electrical component that has only one state, which, if it changes at all, changes linearly. Examples of passive elements include capacitors, inductors, resistors, and transmission lines. For illustrative purposes, specific reference is made to GaAs PIN diodes throughout this disclosure. It should be understood, however, that the invention is not limited to this particular type of compound semiconductor device.
An exemplary prior art GaAs PIN diode is disclosed in detail in U.S. Pat. No. 5,159,296 issued to Nelson and is shown in FIG. 1. The figure shows a compound semiconductor device 100 comprising an active component 101, which, in this case, is a PIN diode 101a, immediately adjacent a passive element 111, which, in this case, is a metallic transmission line 111a used to connect various circuit elements together. The transmission line 111a may be either parasitically coupled to the PIN diode or directly coupled using an additional metal layer. This particular transmission line 111a is a metal layer that is deposited on the surface of the semiconductor substrate 119 and is parasitically coupled to the PIN diode 101a. By configuring the device 100 in this manner, a Schottky junction 113 is formed between the ground plane 105 and the deposited metallic transmission line 111a. This junction can be represented as a diode 115 in series with a resistor 117 leading to ground.
A Schottky barrier is formed when a metal is placed in intimate contact with a semiconductor. A transfer of charge from the semiconductor to the adjoining metal will occur to satisfy the requirement for a continuous Fermi level across the metallurgical junction. The metal will accumulate electrons or holes along its surface and the semiconductor will form a depletion region of the opposite charge. Under ideal conditions, the magnitude of the potential barrier that develops from the transfer of charge is equal to the difference in the work functions between the metal and the semiconductor. In the case of GaAs, the potential barrier is more dependent on the density of surface states than it is on the type of metal that forms the Schottky junction. Thus, for GaAs, the surface state density is significant, which tends to pin the Fermi level such that the potential barrier will approximate 0.8 eV, independent of the Schottky metal. The characteristics of any Schottky junction will largely depend on the surface condition present during the deposition of the metal. Therefore, the processing steps used to prepare the surface for metallization are critical to the device characteristics and the overall-manufacturing yield.
The PIN diodes described above are well suited in applications where the RF signal power and the system potentials are low, such as in automotive applications that use a 12 volt system, since, at relatively low operating voltages, the amount of leakage current that flows from the transmission line 111a to ground is negligible. It is desirable, however, to use these PIN diode configurations in systems using higher energy signals such as missile systems that handle large RF signals and create sizable potentials across the PIN diodes. Unfortunately, when the voltage level increases past a certain breakdown point, a sizable leakage current flows from the junction 113 located between the metal transmission line 111a and the GaAs layer to ground, thereby rendering the circuit useless for normal operation. The applicants recognize that this current leakage is due to the inadequate insulating qualities of the GaAs at higher voltage levels. The semi-conducting nature of the GaAs substrate causes it to behave as a large-value resistor, meaning it is an insulator at lower voltage levels but conducts relatively large currents at higher voltage levels. It has been found the prior art PIN diode switches fail almost immediately and catastrophically when the potential increases to levels of 28-34 volts and above. Accordingly, as used herein, the term “high voltage” refers to potentials greater than 34 volts.
It is desirable to have a device that functions in a manner similar to the prior art devices, but that will withstand the higher voltage levels required by the high-energy signal systems found today.